Which of these numbers is prime? ${9,\ 23,\ 51,\ 56,\ 65}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 23 are 1 and 23. The factors of 51 are 1, 3, 17, and 51. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 65 are 1, 5, 13, and 65. Thus, 23 is a prime number.